Because I can sympathize with your situation, I'm going to try and provide some useful advice. Personally myself I'm saving to get an exhaust system, probably Invidia...unfortunatly living in Canada means I'll be paying at least $800-1300 for a decent exhaust. Check out some of the mufflers from Vibrant, Tom's and AC Autotechnic. Those are the ones I've researched and if you are looking for a nice looking muffler with a good deep sound, any of those should fit the bill. Remember, getting an aftermarket muffler
isn't totally copping out and you'll notice the manufactuers claim "X" to "X" number of horsepower from their mufflers. This is because you can still have piping done to make a complete exhaust and depending on where you go and the material of the piping it could be a viable alternative. Exhaust systems typically make greater gains because of the R&D and high price piping that uses mandrel bent or whatever tubing for more gain.
If you can pick up a cool muffler and are happy with the look and sound, go for it, there is nothing stopping you from getting a real
exhaust down the road. Honestly, I don't even think it's worth getting a cat-back exhaust for the money I'll be paying and the couple extra horsepower but I'm going to do it anyway.
Make sure to check out some of Vibrant and AC's titanium and real carbon fiber mufflers...they look pimp!
Hope any of this helps, if you have anymore questions let me know!